Heretofore, as a developing method in an image forming apparatus using an electrophotographic method, there have been known a single-component developing method using only toner as a developer, and a two-component developing method using toner and carrier.
The single-component developing method is advantageous in aspects of simplification, smaller in size, and a lower cost of an apparatus, while toner degradation is likely to be accelerated due to heavy stress at a control section where toner is charged, and charge receptivity of toner is likely to be lowered. Further, since a toner regulating member or a surface of a developing roller is contaminated by toner or external additives, a charge providing property to toner is decreased to result in shorter service life of the developing device.
In the two-component developing method, since toner is triboelectrically-charged by being mixed with carrier, stress on toner is small, and therefore, the method has an advantage in preventing toner from being degraded. Further, since the surface area of carrier is large, the method is also relatively resistant to contamination with toner or external additives, and is advantageous in terms of longer service life.
However, in the two-component developing method, since the surface of an image carrier is rubbed by a magnetic brush formed of the developer when an electrostatic latent image on the image carrier is developed, traces of the magnetic brush can be produced, and furthermore, carrier is likely to be adhered to the image carrier, resulting in a problem of image defects.
As a developing method to solve a problem of image defects while having a characteristic of a long life of the two-component developing method, there is disclosed a so-called hybrid developing method in which two-component developer is carried on a sleeve roller, and only toner is supplied from the two-component developer to a developing roller, which toner is then used for development (refer to Patent Document 1).
However, in the hybrid developing method, if the gap between the image carrier and the developing roller, that is, a developing nip, varies, the electric field intensity in the developing nip varies, and whereby the degree of development of an electrostatic latent image on an image carrier also varies. For that reason, in order to obtain an appropriate image density, a structure stably securing a uniform gap between the image carrier and the developing roller is required.
Further, in order to form a thin toner layer composed of an appropriate amount of toner on the surface of developing roller, a structure stably securing a uniform gap in the shaft direction between the developing roller and the sleeve roller is required.
To cope with such issues, disclosed is an image forming apparatus which is provided with a rotation shaft holding unit having a pair of openings in which rotation shafts of a magnetic roller and a developing roller are integrally fit so as to be rotatably hold, and the opening of the developing roller side is made swingable around the shaft of the magnetic roller side (refer to Patent Document 2).
Further, disclosed is an image forming apparatus in which a gear is provided on each of a developing roller and a magnetic roller, and driving force is transferred from the magnetic roller to the developing roller through an idling gear, which is held on a rotation shaft holding member which swings around the shaft centers of the developing roller and the magnetic roller, whereby each of gaps among a magnetic roller, a developing roller, and an image carrier is made uniform (refer to Patent Document 3).
In addition, disclosed is an image forming apparatus in which a magnetic roller, a developing roller, and a gap pulley are rotatably held, a movable bearing which holds a developer control plate is provided, and the movable bearing is urged toward the image carrier, whereby each of gaps among a magnetic roller, a developing roller, and an image carrier is made uniform (refer to Patent Document 4).
On the other hand, in the hybrid developing method, a bias voltage is applied between a developing roller and a sleeve roller to transfer only toner to the developing roller, and only toner is made to jump from the surface of the developing roller to a latent image on an image carrier. However, as the speed of image formation becomes faster, it has become difficult for a sufficient amount of toner to jump, which results in a problem of decrease in density of a toner image.
To cope with such problems, there is disclosed an image forming apparatus which can control, even in the case where an image carrier is rotated in a high speed, a decrease in toner density due to such high speed by transferring toner in a plurality of times with arranging a plurality of developing milers (refer to Patent Document 5).